I'm sorry this happened to you. It's heart breaking to watch people we love kill themselves. The one thing I had to learn is that the A has the right to make their own decisions, even if they're bad ones. Your friend has decided to end the friendship, probably a bad decision, but it's hers to make. She has decided to continue drinking, nothing anyone can do about that.
My A villified me to everyone when he was active in his disease. Made me the bad guy, and that was incredibly hurtful. I was the one trying to save him! I finally had to completely detach. It didn't take long for others to see what I had been seeing all alone.
I really wouldn't contact the sister any further. It honestly isn't your place to discuss all this with family, again, it's her right to live her life how she chooses. It also keeps you enmeshed, and gives her another reason to blame you.
Keep your head up. You tried to be a good friend. Now that you're out of the picture, and she's still drinking, she isn't going to have you to blame. Her behavior will become more apparent to others. Find an AlAnon group and try some meetings. You will find alot of support for your situation there.